CuNi 90/10 (UNS C70600) and CuNi 70/30 (UNS C71500) are both copper-nickel alloys built for seawater and corrosion-critical service, and both are available across our full forged fitting range. The difference between them isn't which one is "better" — it's which one matches your flow velocity, mechanical loading and budget.
Composition
| Element | 90/10 (UNS C70600) | 70/30 (UNS C71500) |
|---|---|---|
| Copper (Cu) | 88 – 90% | 65 – 69% |
| Nickel (Ni) | 10 – 11% | 29 – 33% |
| Iron (Fe) | 1.0 – 1.8% | 0.4 – 1.0% |
| Manganese (Mn) | 1.0% max | 1.0% max |
| Equivalent standards | EN CuNi10Fe1Mn, DIN 2.0872, JIS C7060 | EN CuNi30Fe2Mn2, DIN 2.0882, JIS C7150 |
The nickel content is the headline difference, but the iron content matters just as much in practice — both alloys rely on controlled iron to resist erosion-corrosion, and manufacturers hold it within a tight band rather than treating it as incidental.
Mechanical Properties
The higher nickel content in 70/30 delivers meaningfully greater strength — tensile strength typically runs 380–480 MPa versus 275–345 MPa for 90/10. For fittings subjected to higher mechanical loading, pressure cycling, or where a thinner wall section is desirable for weight or space reasons, 70/30's added strength gives more margin.
Corrosion Resistance & Seawater Flow Velocity
Both grades resist general seawater corrosion well, but their tolerance for flow velocity differs — this is usually the deciding factor in fitting selection. CuNi 90/10 is typically rated for seawater velocities up to roughly 3.5 m/s before erosion-corrosion becomes a concern at bends, tees and other turbulence points. CuNi 70/30 extends that margin to roughly 4.5–5 m/s, making it the preferred choice for high-velocity lines, pump discharge piping, and locations where turbulence is unavoidable — such as directly downstream of an elbow or reducer.
Typical Applications
- General seawater cooling piping
- Ballast system piping
- Condenser & heat exchanger tubing systems
- Cost-sensitive, moderate-velocity runs
- High-velocity seawater lines
- Offshore riser & critical process piping
- Desalination plant high-flow sections
- Higher mechanical loading applications
Cost Considerations
Nickel is the more expensive constituent in both alloys, so 70/30's roughly three-times-higher nickel content puts it at a noticeable cost premium over 90/10 — often 20–40% higher depending on current metal prices. For piping sections where 90/10's velocity and strength margins are already comfortable, there's little reason to pay the 70/30 premium. Where velocity or mechanical loading pushes past 90/10's comfortable range, the extra cost of 70/30 is cheaper than an early fitting replacement.
How to Choose
Start with your design flow velocity at the fitting location — not the average pipeline velocity, since bends, reducers and tees see local acceleration well above the bulk flow rate. If that local velocity stays comfortably under roughly 3 m/s, 90/10 is almost always the right, more economical choice. If it approaches or exceeds that threshold, or the fitting sees significant mechanical or pressure cycling, 70/30 is the safer specification.
In short: 90/10 covers the majority of general seawater piping economically; 70/30 earns its premium in high-velocity, high-load or critical-service locations. We manufacture our full fitting range in both grades, so the choice comes down to your application, not our capability.